The present invention is directed to an RF power amplifier system.
RF power amplifiers are known in the art for use in amplifying RF signals for broadcasting purposes, including radio and television. The amplifiers may be employed for broadcasting analog television signals, sometimes referred to as NTSC modulated signals, or digital signals, sometimes known as HDTV or DTV signals.
In the amplification of such RF signals, it is common to split the RF signal to be amplified into portions and then to amplify each portion and re-combine the amplified portions with a target to deliver an RF signal more powerful than a single power amplifier PA portion can produce. This will provide an amplified RF signal for application to an antenna system.
As will be brought out hereinafter with reference to FIGS. 1-9 there may be some level unbalance between RF signals generated by the PA portions resulting from the structures employed in the signal splitter. This unbalance will increase re-combining losses of RF signals generated by the PA portions and therefore, reduce the amount of RF signal delivered by the power combiner to the antenna system. Additionally, this unbalance will also exaggerate intermodulation (IMD) products in the sidebands associated with the transmitted channel.
It has been determined that in order to mitigate the effects related to RF signal unbalance the input splitter should have a broadband frequency response with minimum imbalance between channels while the combiner may remain narrow banded featuring the simplest structure and associated unbalance between channels to be able to exhibit the shortest path to the output and consequently the lowest signal loss.
In accordance with the invention, an RF wideband amplifier system is provided that includes an M way splitter for receiving an RF input signal and splitting same into M RF signals for respective application to M power amplifier modules PAM-1 to PAM-M that amplify the M signals and apply the amplified M signals to an M way combiner that applies an amplified RF signal to a load. A main controller provides an automatic level control reference signal, representative of the desired output power level of each of the power amplifier modules. Each power amplifier module includes an 2*N way balanced splitter that receives one of the M RF signals and splits the signal into 2*N signal portions; 2*N RF amplifiers that respectively receive the 2*N signal portions and amplify same and provide therefrom 2*N amplified signal portions; an 2*N way RF combiner that receives and combines the amplified RF signal portions and provides therefrom a combined amplified output RF signal; a power detector that provides a power signal representative of the level of the output power of the power amplifier module; a difference circuit that provides an attenuation control signal having a value in accordance with the difference in values of the power signal and the reference signal; and, a variable adjuster that adjusts the magnitude of the one of the M RF signals in accordance with the difference.